The present invention relates to a thermal disconnect for removing mechanical connection between a prime mover and a load in response to overheating of the load.
It is known to run devices such as generators, constant speed drive mechanisms, and pumps from shafts or spools of prime movers, such as, for example, gas turbine aeronautical engines. Although the load device should be reliable, there is always a possibility that it will experience abnormal operating conditions or will enter a failure mode wherein damage to the load may occur, with the possibility that such damage may result in abnormally high torques being required from the prime mover, possibly to the extent that its integrity will be compromised.
U.S. Pat. No. 5,103,949 discloses a disconnection device in which a drive shaft carries a threaded portion and a plunger is arranged to move in from the side of the drive shaft to engage the threaded portion, thereby causing the screwing action between the plunger and the threaded portion to displace the shaft axially such that it becomes disconnected from a prime mover.
U.S. Pat. No. 4,042,088 similarly has a drive shaft which engages a prime mover via a castellated connection region. The drive shaft also carries a helically threaded portion and a disconnect plunger is arranged to move in from the side of the drive shaft to engage the threaded portion to cause the shaft to move axially so as to disconnect the shaft from the prime mover.
U.S. Pat. No. 4,989,707 discloses a similar arrangement in which a element moves in from the side of a drive interface between input and output shafts in order to engage a threaded portion of a ring in order to cause disconnection between the shafts.
Each of these prior art arrangements suffers from the problem that the shafts will, in general, be rotating rapidly and that the teeth on the control element which moves radially inward to engage the screw thread must engage the thread rapidly before they become damaged or sheared off.
U.S. Pat. No. 4,086,991 discloses a disconnect coupling in which helical splines are used to connect a coupling shaft to a driven member so as to transmit torque to the driven member. The helical splines are carried on an axially movable shaft and are arranged such that the transmission of torque to the driven member acts to urge the coupling shaft and driven member to move to a disengaged position. In normal use, this movement to a disengaged position is inhibited by the provision of a fusible element, such as a eutectic pellet. Such pellets are generally of a soft material and the crushing of the pellet under a compressive load is a well recognised problem, see for example, U.S. Pat. No. 4,271,947 wherein the pellet is manufactured with wire strands therein in order to give it additional mechanical strength. Because of the use of helical splines in U.S. Pat. No. 4,086,991 the crushing force acting on the eutectic pellet varies as a function of the torque transmitted through the disconnect coupling. In particular, the pellet must be able to withstand the crushing load at full torque transfer without suffering deformation. This increase in material in the pellet means that the pellet has an increased mass and thermal capacity, and as such the rate at which the pellet warms is reduced, thereby leading to a potential slowing of the decoupling mechanism. Another problem with the system described in U.S. Pat. No. 4,086,991 is that decoupling between the shaft and the load may not occur when the shaft is lightly loaded. This is significant since the load may be a generator and it is conceivable that the generator itself may not fail, but that the cooling system for the generator might fail, thereby resulting in the need to disconnect the generator in order to prevent damage to it even when the generator is lightly loaded.
U.S. Pat. No. 4,271,947 discloses an arrangement in which two axially aligned shafts engage each other via coaxial gears having teeth extending in the axial direction. A compression spring extends between the gears and urges an axially displaceable one of the gears to move away from the axially fixed gear. A fusible element having strengthening filaments therein acts to resist both the force of the compression spring and the axial forces resulting from torque transfer via the inclined surfaces of the gear teeth.
According to the present invention there is provided a drive disconnect device comprising:
an input element having a first axis and a first connection region carrying first engagement elements;
an output element having a second axis and a second connection region carrying second engagement elements;
restraint means; and
biasing means;
wherein the input element and output element can undergo relative axial displacement with respect to one another between a coupled position where the first engagement elements cooperate with the second engagement elements such that the input element is drivingly connected to the output element, and a decoupled position where the first and second engagement elements are disengaged from one another, the biaising means acts to urge the input and output elements towards the decoupled position and the restraint means serves to resist the urging of the biaising means until the temperature of the restraint means exceeds a predetermined value, and wherein the engagement elements extend only axially along at least one of the input and output elements.
It is thus possible to ensure that the contacting surfaces which serve to transmit load between the input element and the output element engage each other on a surface with which the first axis or the second axis is parallel.
It is thus possible to ensure that the force acting on the restraint means does not vary as a function of the torque being transmitted by the drive disconnect coupling.
Preferably the first and second engagement elements are splines. The first engagement elements may extend radially outward from the input element and the second engagement elements may extend radially inward from the output element. The input element is preferably radially smaller than the output element such that it can be partially disposed within the output element and movable axially with respect to the output element.
Preferably the input element carries a third engagement region for releasably engaging with a drive shaft of a prime mover, or with a coupling element connected to a drive shaft.
Preferably the coupling element acts to inhibit lubricant loss from around the drive shaft when the drive disconnect device is in its disconnected state.
Preferably the third engagement region cooperates with a fourth engagement region carried in the coupling element such that the input element is drivingly connected to the coupling element when the input element is coupled to the output element.
Advantageously the third and fourth engagement regions disengage from one another when the input element decouples from the output element. A further biaising device may be provided to urge the input element to drivingly disconnect from the coupling element.
Preferably the biasing means comprises at least one spring. The or each spring may be a wave spring, and where multiple wave springs are provided these may be stacked in a crest to crest fashion. This is the advantage of providing a spring configuration which when in its compressed state takes up a relatively small amount of space in the axial direction, but which can give a relatively high degree of expansion.
Advantageously the restraint means is formed from a material, such as a eutectic mixture or solder whose melting temperature is well controlled. The material is selected such that it will melt at a temperature that will be attained by the lubricant if the device driven by the coupling fails or overheats. Preferably a pellet or column of the eutectic material is formed and is positioned such that it either directly or indirectly bears the load provided by the biasing means. Advantageously the eutectic is placed within a substantially sealed container such that, as it melts, it is restrained from coming into contact with lubricants used in the vicinity of the drive disconnect device.
In a preferred embodiment, the eutectic mixture is provided in the form of a cylinder (ie a hollow column). The cylinder of eutectic material is advantageously in contact with a container. The container may, for example have the profile of a disk having two upstanding walls disposed at different radii from the centre of the disk. The container may itself then be positioned such that the base and/or walls thereof are directly bathed in lubricant or in good thermal contact with components that themselves are bathed in lubricant, such as oil. This arrangement helps to ensure good heat transfer between the lubricant and the eutectic restraint means. This is advantageous to ensure rapid operation of the disconnect mechanism.
Thus the individual elements of profiling the connection between the input and output elements so as to remove axial force acting on the eutectic (thereby allowing less eutectic to be used to obtain the necessary physical strength) and the good thermal contact between the eutectic and the lubricant each give significant operational advantages and together provide a thermal disconnect mechanism having significantly reduced disconnect period, ie it disconnects much more quickly because the heat from the lubricant is transferred more quickly to the eutectic.